The present invention relates to methods and systems for controlling the power level of high frequency signals, and more particularly to RF telemetry transmitters for efficiently communicating with and powering an implanted stimulator device (e.g., an implanted cochlear stimulator (ICS)).
Cochlear implant technology is well known and has been successfully used to enable individuals to hear, whereas other hearing assist devices, such as hearing aids and head phone amplifiers, have failed. Generally, cochlear implant systems include an external unit and an implanted device. The external unit usually includes a power source (e.g., a battery), where the implanted device may not. The implanted device may receive power from the external unit by way of an inductive or radio frequency (RF) link. To transfer power from the external unit to the implanted device, the external unit and implanted device may each include a coil. Although these coils are not directly connected, a high frequency carrier signal, which is applied to the external device coil, is coupled to the implanted device coil. This coupling is akin to the flux coupling seen in transformers. That is, even though the primary and secondary windings are not directly coupled to each other, an AC signal applied to the primary winding is also applied to the secondary winding by virtue of the flux coupling. In an ICS system, the carrier signal is received by the implanted coupling and then rectified into a DC signal for powering the implanted device.
Control and/or data signals may be transmitted to the implanted device by applying a predetermined modulation signal to the carrier signal. For example, acoustic signals received and processed by the external device may be converted into electrical signals (e.g., a digital pulse stream), which may provide a basis for the modulation applied to the carrier signal.
Cochlear implant systems, like many other electronic systems, are constantly subject to ever stringent design criteria such as smaller size requirements, greater power efficiency, and lower costs. One way to address each of the foregoing design criteria, and others not mentioned, is to increase the efficiency of power conversion and transfer from the external unit to the implanted unit. Traditional power conversion and transfer techniques, although are able to provide power to the implanted device, do not completely meet the stringent criteria. As a result, the external unit of the cochlear implant systems may require bulky housings, large power requirements, and frequent replacement or charging of batteries.
One example of a known power conversion technique uses a class D, E/F, G, H, or S transmitter in combination with a voltage regulator (e.g., a switching regulator). The voltage regulator controls the transmitter supply voltage, which control is responsible for adjusting the power output of the transmitter. A drawback with this technique is that the voltage regulator requires additional circuitry such as control circuitry and discrete components such as inductors and capacitors to operate. These additional components add costs, consume additional power, and occupy extra space.
Another power conversion technique eliminates the need to use a voltage regulator to control the transmitter power output by using a pulse width modulation (PWM) technique (or duty cycle control) to adjust the magnitude of the carrier signal. One drawback of using PWM to control the transmitter power output is that both even and odd higher order harmonics may be imposed on the carrier signal. As is known in the art, harmonics represent unwanted components of a signal (e.g., a carrier signal) that are typically produced in high frequency applications. The production of both even and odd harmonics places a substantial burden on filtering circuitry because if the higher order harmonics currents are not suppressed, these harmonic currents can decrease the power efficiency of the transmitter circuitry. The production of undesirable second harmonics increases the steepness of the suppression filter circuits, which generally increases the order of the filter and the inherent losses in such filters. The radiation of the harmonic components may generate EMI (e.g., Electro-Magnetic Interference), which is often prohibited or regulated. Moreover, another drawback is that the control of the duty cycle may be difficult as it approaches zero, thus potentially preventing accurate control of power across the entire available range of power that can be transmitted on the carrier signal.
Because many cochlear implant devices are implemented in relatively small behind-the-ear units, space and power are at a premium. Furthermore, as cochlear implant devices advance, other components such as digital processing circuitry may require increased levels of power and space. Thus, there is a need for a high frequency transmitter circuit that is both compact and efficient.